Add another schema to Oracle Apex application

Case One
The following procedure is good if you have admin account for Internal workspace.
1. Log into Internal workspace with admin account.
2. Go to Manage Workspace

3. Choose "Manage Workspace to Schema Assignments" under "Workspace Actions"
 
4. Add Schema >> Existing >> Select the workspace, to which you are adding schema >> choose the schema you want to add

Case Two
If you have Apex application admin account
1. Go to the application, to which you want to add schema.
2. Click administration
3. Choose manage service

4. Make a service request
 
5. Request Schema >> fill in the content accordingly Wait for Apex administration to approve your request.


Now, we have added the schema to Apex application, there is two more things we need to do:
1) granting select previlege to your application PARSING_SCHEMA. The parsing schema is the schema you defined when creating or importing an Apex application. An internal workspace administrator could find it for you if you do not know.

connect / as sysdba
grant select on schema_name.table_name to PARSING_SCHEMA;

2) make sure you use fully qualified table names, i.e. prefix the object name with the schema name, when querying tables from non-parsing_schema.